1. Field of the Invention
This invention relates to novel epoxy resin curing agents which afford reduced shrinkage to the epoxy resin on curing to a thermoset material.
2. Description of the Prior Art
A general concern of polymerization is the volume reduction resulting in the development of internal stresses in a polymeric material. For a thermoplastic this concern of polymerization shrinkage may not be as serious as in thermosets due to the fact that a final fabrication step is required to turn a polymer into a useful material, which usually relieves the build-up of internal stresses. However, a thermoset cannot be further fabricated to release the internal stress through a remelt process. As a result of the entrapped internal stresses in a thermoset, the molded part may experience premature failure when exposed to normal environmental conditions. This phenomenon has been shown in several application areas such as the reduction of adhesion, the failure of interfacial bonding of a matrix and fibers in a composite, and the cracking of a casting compound. Therefore, a thermoset with a zero shrinkage is desirable in many important industrial applications.
Ring opening polymerization usually involves less shrinkage than simple addition polymerization. If the ring is large enough, no shrinkage should be involved in the polymerization. Based on this concept. a bicyclic compound, a ketal-lactone Structure (1), was prepared and polymerized with either boron trifluoride or a base to a polyester without change in volume [Bailey et al-"Polymerization with Expansion Volume" in ACS Symposium Series, "Ring Opening Polymerization", p. 38, vol. 59 (1977)]: ##STR3## The spiroorthocarbonate first invented by Bailey has been seriously considered for development into a commercial product for coatings, adhesives and related polymeric materials. The materials were based on a blend of epoxy resins and the spiroorthocarbonate. It was concluded that the corrosion resistance of steel was significantly improved by the coating containing polymerized spiroorthocarbonate due to the reduced shrinkage, and hence, internal stress (M. S. Cohen et al, Polymeric Materials Science and Engineering, vol. 54, 12 (1986).
A compund having a Structure (2) was prepared simply by reacting a 2-isopropyl-2-oxazoline and maleic anhydride in ethylenedichloride at reflux conditions [R. Nehring and W. Seeliger, Angew. Chem. internat. Edit., vol. 9, 460 (1970)]: ##STR4## The product was described to undergo thermal polymerization when heated at 180.degree. to 200.degree. C. for 20 minutes in the absence of moisture. The reaction produces a poly(ester-imide), a transparent plastic of high molecular weight which is soluble in a number of solvents such as DMF, dioxane, DMSO, ethylenedichloride and pyridine [R. Nehring and W. Seeliger, Angew. Chem. Intern. Edit., vol. 9, 461 (1970)].